(1) Field of the Invention:
The present invention relates to a method and an apparatus for producing hollow metal ingots. More particularly, the invention relates to a method for casting materials, that is, hollow metal ingots (hereinafter, typical "steel ingots" will be discussed by way of example), used for the production of cylindrical forged steel articles such as pressure vessel materials, oversized ring materials and the like as well as an apparatus used for performing the above method.
(2) Related Art Statement:
Recently, uses of hollow steel ingots have been largely expanded, and with this expansion, demands for shapes and qualities of the hollow steel ingots required have become severer and diversified. For instance, there are demands for producing large size articles exceeding 300 tons and articles having no inverse V-shaped segregation lines in the inner surface thereof.
It is now not so difficult to produce such hollow steel ingots themselves. For instance, there are known the following producing techniques:
(1) By using a metallic cylinder as an outer tube to be brought into contact with a molten steel and employing a solid core or a core of a hollow metallic cylinder inside the outer tube, hollow steel ingots are produced while a cooling fluid such as air or steam is flown into the core (see British patent No. 52 05 98).
(2) A core consisting of a cylindrical steel pipe and a cylindrical refractory member formed contacting the inner wall of the cylindrical steel pipe is placed at the center of a mold positioned on a stool, and hollow steel ingots are produced by pouring a molten steel between the mold and the core (see Japanese patent application laid-open No. 54-117,326).
Since the above prior art techniques give easy formation of the core and good cooling performance of the core, it can be said that they are excellent techniques. However, since the demands have been recently not only getting severer and severer for the quality of hollow steel ingots but also the size of the steel ingots has become greater, it is an actual situation that such tendencies cannot be coped with by the prior art techniques. That is, with increase in the size of the steel ingots, it has become difficult to produce hollow metallic cylinders which can withstand the static pressure of the molten steel, be appropriately buckled and deformed and at the same time still maintain a necessary hollow shape against the succeeding pressure. Further, when the size of the steel ingots become larger, cooling of the steel ingots from the inside of the core becomes insufficient. As a result, inverse V-shaped segregation is liable to appear, so that there occurs a quality problem with respect to steel ingots such as materials for atomic energy generation plant for which severer quality is demanded.
For instance, when the thickness of the metallic cylinder itself forming the outer shell of the core is increased to cope with the oversizing of the steel ingots and demands for the quality thereof, the cooling power must be strengthened. On the other hand, fatal cracks occur in the inner surface of the steel ingots if buckling is not produced. If the thickness of the metallic cylinder of the core is reduced, cracking at the inner surface of the steel ingot can be avoided owing to an appropriate degree of buckling. However, there is a danger that the core is crushed because the buckling amount may be beyond the control. With respect to this, if the buckling of the core is intended to be suppressed midway, it is necessary to install an obstacle between the metallic cylinder and a cooling fluid supply system. Consequently, sufficient cooling cannot be performed.
Further, it is known to use water as a cooling fluid. Although in this case the cooling effect is improved, it makes the deformation of the metallic cylinder difficult and there remains a fatal problem in safety. Thus, this technique is not practical.